I keep small bottles of spring water around just for this. I've got my system down that I can take a reading at the end of the boil and know how many if any bottles I have to put in. Just yesterday, 10 gallon batch came in a little high, 2 bottles...dead on. No worries.

Remember though that measuring the OG accurately on fresh AG is tough. I used to get all worried about the OG too. But now I just use brewing software and double check the weights of the grain. I actually find a refraction ether more reliable for checking wort.
You could wait to see the FG too or instead and adjust with water from there. But approaching 1.070 is a concern for some yeast strains, so maybe this was wise insurance.
I'd be interested to hear how your FG turned out.

I've used brewing software for my calculations. I think I came in a bit high because I didn't think the software was giving me accurate mash measurements so I decided to tinker with things a bit (lesson learned)
I've measured my BG and OG at the same temp.
The math I did in my head to add the extra bottled water was spot-on though, as it ended up at exactly what I aimed for at 1.060